The present invention relates to a technique regarding a range finder capable of taking three-dimensional information of a subject (namely, a three-dimensional camera capable of measuring a range image).
FIG. 21 is a diagram for showing the structure of a conventional range finder. In FIG. 21, a reference numeral 51 denotes a camera, reference numerals 52a and 52b denote light sources, a reference numeral 55 denotes a light source control unit and a reference numeral 56 denotes a distance calculation unit. The light source control unit 55 makes the light sources 52a and 52b alternately emit light every field cycle in synchronization with a vertical synchronizing signal of the camera 51.
At this point, it is assumed that the optical center of the camera lies at the origin with the optical axis direction of the camera set as the Z-axis, the horizontal direction set as the X-axis and the vertical direction set as the Y-axis, that the direction of a viewing point from the light sources is at an angle φ against the X-axis, that the direction of the viewing point from the camera is at an angle θ against the X-axis, and that the light sources are positioned at (0, −D), namely, the base line length is D. The depth value Z of the viewing point P is calculated in accordance with the principle of trianglation calculation as follows:Z=Dtanθtanφ/(tanθ−tanφ)  (1)In order to obtain the angle φ, predetermined light patterns are projected by the light sources 52a and 52b. 
As the light sources 52a and 52b, for example, flash light sources 57 and 58 such as a xenon flash lamp are longitudinally disposed with reflector plates 57 and 58 disposed behind to be shifted in the lateral direction as shown in FIG. 22A. FIG. 22B is a plan view of the light sources of FIG. 22A. The light sources 52a and 52b radiate light in ranges A and B, respectively.
FIG. 23 is a diagram for showing light patterns radiated from the light sources 52a and 52b. In FIG. 23, the brightness obtained by projecting the light on a virtual screen Y is indicated along a direction of an arrow shown in the drawing. Specifically, the light projected from each of the light sources 52a and 52b has a characteristic that it is brightest on the center axis and is darker toward the periphery. Such a characteristic is exhibited because the reflector plates 59 and 60 each in the shape of a semi-cylinder are respectively disposed behind the flash light sources 57 and 58. Also, since the reflector plates 59 and 60 are shifted laterally in their directions, the projection ranges of the light sources 52a and 52b partially overlap each other.
FIG. 24 is a diagram for showing the relationship between the light projection angle φ in an H direction of FIG. 23 and the light intensity. The H direction accords with the direction of a crossing line between the virtual screen Y and one optional plane S among a plurality of planes each including the light source center and the lens center. In a region α of FIG. 24, one of the light patterns projected from the light sources 52a and 52b is bright relatively on the right hand side and the other is bright relatively on the left hand side, whereas the brightness of the light pattern is varied also along the height direction (Y-axis direction).
FIG. 25 is a graph for showing the relationship between the light intensity ratio between the two kinds of projected light in the region a of FIG. 24 and the light projection angle φ. As shown in FIG. 25, the light intensity ratio and the angle φ are in a one-to-one corresponding relationship in the region α.
In order to measure a distance, the two kinds of light patterns are alternately projected on a flat plane vertically provided so as to face the light sources at a predetermined distance and reflected light is taken by the camera 1, so that data of the relationship between the light intensity ratio and the light projection angle as shown in FIG. 25 can be previously obtained with respect to each Y-coordinate (corresponding to a Y-coordinate on the CCD). A data with respect to each Y-coordinate means a data with respect to each of the plural planes including the light source center and the lens center. Also, when the light sources 52a and 52b are disposed so that a line extending between the lens center of the camera 51 and the light sources 52a and 52b can be parallel to the X-axis of the CCD camera face, a distance can be accurately calculated by using the data of the relationship between the light intensity ratio and the light projection angle determined with respect to each Y-coordinate.
Assuming that a point P of FIG. 21 is the viewing point, the angle φ of the point P from the light source is measured by using the brightness ratio of the point P obtained images taken with the two kinds of light patterns projected and the relationship as shown in FIG. 25 corresponding to the Y-coordinate of the point P. Furthermore, the angle θ of the point P from the camera is determined on the basis of the position in the image (namely, the pixel coordinate values of the point P) and camera parameters (such as the focal length and the position of the optical center of the lens system). Then, the distance is calculated in accordance with the equation (1) based on these two angles φ and θ and the distance (base line length) D between the position of the light sources and the position of the optical center of the camera.
In this manner, when the light sources for generating the light patterns that are monotonically increased/decreased in accordance with the projection direction as in the region α of FIG. 24 are used, the three-dimensional measurement of a subject can be simply carried out.
However, in the conventional structure, the xenon flash lamp, which has a life of merely approximately 5000 stable emissions, is used as the light source. Therefore, when the range finder is used for a long period of time, maintenance such as exchange of the lamp should be frequently conducted. Also, the stability of the quantity of light emitted by the flash lamp is merely several %, and hence, higher measurement accuracy cannot be obtained.
Furthermore, a light source with a long life is, for example, an LED (light emitting diode), but the quantity of light emitted by one LED is small. Therefore, when the LED is singly used, the light quantity is so insufficient that the three-dimensional measurement cannot be stably carried out.
Moreover, since the projected light patterns are determined in accordance with the shapes of the reflector plates in the conventional structure, merely one set of light patterns can be generated in principle.